Change the implementation for arm926 to relocate the code to
an arbitrary address in RAM.

Adapt the TX25 (i.MX25), magnesium board to test the changes.

On the tx25 board TEXT_BASE is set to the final relocation
address to prevent one more copying of u-boot code
when relocating. More info see:
doc/README.arm-relocation

da850 board:
Tested-by: Ben Gardiner <bengardiner@nanometrics.ca>

Portions of this work were supported by funding from
the CE Linux Forum.

Signed-off-by: Heiko Schocher <hs@denx.de>
Cc: Ben Gardiner <bengardiner@nanometrics.ca>